The invention relates to a crystal-controlled line-voltage ballast for compact RF fluorescent lamps and more particularly to an integral high efficiency ballast circuit for operating electrodeless fluorescent lamps.
In recent years, lamp ballasts operating in the 100-150 kHz region have been successfully developed to start and run solenoidal electrodeless fluorescent (SEF) lamps. In such lamps a ferrite or air core coil is used to couple RF energy to the lamp discharge. With recent advances in high voltage FET (field effect transistor) technology it is possible to produce sinewave power at line voltage levels with high efficiency and at frequencies extending into the megahertz range. Higher frequency operation makes possible smaller, lighter weight, lower cost ballast circuitry which is especially attractive for an integral ballast and lamp package product.
Electromagnetic interference (EMI) generated by an SEF ballast includes the fundamental and its harmonic frequencies and is produced continuously while the SEF lamp is operating, and not momentarily merely to start the lamp as in conventional fluorescent lamp ballasts. Since there is no frequency band assigned to industrial, scientific and medical (ISM) use which includes the 100-150 kHz region, a ballast and lamp package operating in this region must be very well shielded to comply with FCC regulations governing electromagnetic radiation. Operation at a frequency assigned for ISM use would avoid the need to seek FCC approval to operate lamps at some frequency not presently set aside for such a commercial product. It would remain necessary to minimize RF interference with other electronic devices, such as radio and television receivers, which can easily be used within distances only a few inches from the lamp.
In view of the advances in high voltage FET technology, and the advantages inherent therefrom, as hereinbefore stated, an integral SEF lamp ballast which would operate at higher frequencies than those heretofore developed would be desirable. The physical size of the ballast circuitry would be small enough so that it could be conveniently and integrally packaged with an SEF lamp without excessively increasing the size of the lamp. Further, the ballast circuitry would be inexpensive relative to a low frequency ballast, and high volume production SEF lamps would become more economically attractive.
The efficiency (AC line power to RF power conversion) of the ballast should be high, say greater than about 70 percent, to minimize problems of heat sinking and thermal management. Further, the efficiency should not vary greatly over the range of expected line voltages, thus enabling the use of a smaller DC filter capacitance whereby the input power factor of the ballast is improved. Sufficient power must be provided to the lamp from the ballast to assure starting and restarting of the lamp over the full range of expected ambient temperature. Further, provision for dimming the lamp and for readily adjusting the output impedance of the ballast circuitry to accommodate impedance variations among lamps of a similar type is desirable. Also, the output frequency of the ballast should be maintained relatively constant regardless of variations in impedance of the load (SEF lamp), supply voltage or temperature of the ballast.
Accordingly, it is an object of the present invention to provide a ballast for an SEF lamp wherein the output frequency of the ballast is in an ISM frequency band.
Another object is to provide a ballast capable of being integrally packaged with an SEF lamp.
Still another object is to provide a ballast having a dimmer control for an SEF lamp.
Yet another object is to provide a high efficiency ballast for an SEF lamp.
A further object is to provide a ballast for an SEF lamp wherein the output frequency of the ballast does not significantly vary as a function of load impedance, supply voltage and temperature of the ballast.